This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C xc2xa7119 from an application entitled Wavelength Division Multiplexing Erbium Doped Fiber Amplifier Which Outputs Constant Power Per Channel, And Amplifying Method Thereof earlier filed in the Korean Industrial Property Office on Oct. 7, 1998, and there duly assigned Serial No. 98-41860 by that Office.
1. Field of the Invention
The present invention relates to a wavelength division multiplexing erbium doped fiber amplifier (WDM-EDFA) having constant output power per channel and an amplifying method thereof, and more particularly, to a WDM-EDFA which keeps constant output power for each channel when the number of channels is changed.
2. Description of the Related Art
WDM-EDFAs output a constant output power per channel (hereinafter referred to as COP), depending on the wavelength band and the power of received signal light or the number of channels in a gain-flattened wavelength area of a given EDFA. For example, assuming that the power of signal light amplified and output by the WDM-EDFA is required to be +5d Bm/channel when the central wavelength interval of received signal light for each channel is 0.8 nm and the power of the received signal light is xe2x88x9220 dBm/channel in a 16 channel WDM optical transmission system, a controller of the WDM-EDFA receives information about the number of channels from the optical transmission system and controls the WDM-EDFA in accordance with the required output conditions of the WDM-EDFA.
FIG. 1 is a configuration view of an exemplary WDM-EDFA. The amplifier of FIG. 1 includes a first optical coupler 102, a first photoelectric converter 104, a first isolator 106, an amplification unit 108, a second isolator 110, a second optical coupler 112, a second photoelectric converter 114, and a controller 116. Reference numerals 100 and 118 denote an input connector and an output connector, respectively.
In the operation of the conventional WDM-EDFA, the first optical coupler 102 divides signal light received through the input connector 100 at a ratio of 1 to 99. The first photoelectric converter 104 converts the 1% of signal light which was divided from the signal light by the first optical coupler 102, into an electrical signal, and transmits the electrical signal to the controller 116. The controller 116 measures the amplitude of the converted signal and controls the amplification level of the amplification unit 108. The amplification unit 108 amplifies the 99% of signal light which was divided from the signal light by the first optical coupler 102. The second optical coupler 112 divides signal light amplified by the amplification unit 108 at a ratio of 1 to 99. The second photoelectric converter 114 converts the 1% of signal light which was divided from the signal light by the second optical coupler 112 into an electrical signal and outputs the electrical signal to the controller 116. The controller 116 measures the amplitude of the signal converted by the second photoelectric converter 114 and controls the amplification level of the amplification unit 108. The first isolator 106 prevents amplified spontaneous emission (ASE) generated by the amplification unit 108 from degrading the amplification efficiency by being reflected by the first optical coupler 102 and reapplied to the amplification unit 108. Similarly, the second isolator 110 prevents ASE generated by the amplification unit 108 from degrading the amplification efficiency by being reflected by the second optical coupler 112 and reapplied to the amplification unit 108. However, when adding/dropping of channels occurs in a WDM optical transmission system, signal light is instantaneously transmitted via an optical fiber, but channel information is transmitted after a predetermined delay time. FIG. 2 shows an example of a variation in the output per channel of a WDM-EDFA according to the transmission time difference between signal light and control light including channel information.
If the number of channels is changed from 16 to 8, the power of incident signal light is reduced to a power for 8 channel signal light, so that the WDM-EDFA, which makes COP control suitable for 16 channels as shown in a section 200, increases the output per channel to keep the total output power constant. At this time, overshoot such as that in a section 202 occurs. That is, channel information associated with a variation in the number of channels reaches the controller after a predetermined delay time, so that the time delay causes a transient effect such as overshoot. Reference numeral 204 denotes a section where amplification control with respect to 8 channels is performed.
An objective of the present invention is to provide a WDM-EDFA which keeps the output power per channel constant even when the state of channels is changed, by installing a power control portion for controlling the power of signal light in the front portion of the WDM-EDFA, and an amplification method thereof.
To achieve the above objective, the present invention provides a wavelength division multiplexing erbium doped fiber amplifier (WDM-EDFA) having constant output power per channel, which amplifies received signal light of a plurality of channels which are multiplexed, the WDM-EDFA including: a power control unit for generating dummy light whose output power is controlled according to a first control value, and controlling the power of received signal light by coupling the dummy light to the received signal light; an optical fiber amplification unit for amplifying signal light power-controlled by the power control unit, using pumping light generated according to a second control value; and a controller for supplying the first control value to the power control unit to obtain dummy light power corresponding to the difference between the power value of the received signal light and a predetermined target power value, and supplying the second control value to the optical fiber amplification unit to obtain pumping light power required to amplify the received signal light.
To achieve the above objective, the present invention provides a method of amplifying a wavelength division multiplexing erbium doped fiber, including the steps of: measuring the power of received signal light and obtaining the difference between the measured power value and a predetermined target power value; generating dummy light having output power as large as the obtained difference; coupling the generated dummy light to the received signal light and outputting the coupled light; and amplifying the coupled light.